Computer graphics processing and selective visual display system – Display driving control circuitry – Display power source
Reexamination Certificate
2001-04-27
2004-04-20
Mengistu, Amare (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
Display power source
C345S204000
Reexamination Certificate
active
06724380
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a contrast control circuit for a display apparatus, and more particularly, to a contrast control circuit for a display apparatus, which receives a first voltage of positive polarity adjusted by a user, and outputs a second voltage of negative polarity for controlling the contrast of a liquid crystal display panel.
2. Description of the Related Art
Referring to
FIG. 1
, a liquid crystal display apparatus
1
, which is one of general display apparatuses, includes a liquid crystal display panel
11
and a driving apparatus thereof.
In the liquid crystal display panel
11
, signal electrode lines SL
1
, SL
2
, . . . , SLm are parallel arranged on cells of liquid crystal (LC), and scan electrode lines CL
1
, CL
2
, . . . , CLn are parallel arranged under the LC cells to be orthogonal to the signal electrode lines SL
1
, SL
2
, . . . , SLm. The scan electrode lines CL
1
, CL
2
, . . . , CLn and the signal electrode lines SL
1
, SL
2
, . . . , SLm are made of a transparent conductive material, for example, ITO (indium tin oxide).
The driving apparatus includes a controller
14
, a segment driver
12
, a modulation-signal generator
141
and a common driver
13
. The controller
14
processes a video signal Sc from a host computer, e.g., a notebook computer, and generates a data signal DATA, a shift clock signal SCK, a frame signal FLM and a latch clock signal LCK.
The segment driver
12
makes the input data signal DATA stand by in the respective signal electrode lines SL
1
, SL
2
, . . . , SLm according to the shift clock signal SCK. Also, a signal voltage corresponding to the data signal DATA standing by according to the latch clock signal, is applied to the respective signal electrode lines SL
1
, SL
2
, . . . , SLm. Here, a circuit of generating bias voltages VBS supplied to the segment driver
12
is shown in FIG.
2
.
The frame signal FLM indicates starting of a frame. The modulation-signal generator
141
divides the frequency of the latch clock signal LCK and generates a modulation signal. The generated modulation signal controls the polarity of voltages output from the segment driver
12
and the common driver
13
.
The common driver sequentially applies the corresponding scan voltages to the respective scan electrode lines CL
1
, CL
2
, . . . , CLn by controlling the latch clock signal LCK, the frame signal FLM and the modulation signal. Accordingly, the arrangement of liquid crystal (LC) of a to-be-displayed cell changes and light is transmitted or blocked. Here, a circuit of generating bias voltages VBC supplied to the common driver
12
is shown in FIG.
2
.
Referring to
FIG. 2
, the circuit of generating bias voltages applied to the common driver (
13
of
FIG. 1
) and the segment driver (
12
of FIG.
2
), includes a contrast control circuit
151
and a bias-voltage generator
15
. The contrast control circuit
151
receives a first voltage of positive polarity adjusted by a user and outputs a second voltage VOUT of negative polarity for controlling the contrast of a liquid crystal display panel. The bias-voltage generator
15
produces bias voltages VBC and VBS applied to the common driver
13
and the segment driver
12
according to the output voltage VOUT of the contrast control circuit
151
.
FIG. 3
is a detailed circuit diagram of the contrast control circuit
151
of the conventional liquid crystal display apparatus.
Referring to
FIG. 3
, the positive voltage applied to an input port
2
of an operation amplifier (OP-AMP) is determined by a positive supply voltage VCC and eighth, ninth and tenth resistors R
8
, R
9
and R
10
. Reference symbol C represents a capacitor for removing radio frequency noise. The resistance of the ninth resistor R
9
varies by user's manipulation. For example, the positive voltage applied to the input port
2
of the OP-AMP varies in the range from 0 to 3 V.
The OP-AMP operable by positive and negative supply voltages VCC and VEE, outputs a negative voltage VOUT, the absolute value of which is proportional to the positive voltage applied to the input port
2
. The absolute value of the output voltage VOUT is proportional to the combined resistance value of eleventh, twelfth and thirteenth resistors R
11
, R
12
and R
13
. The negative voltage VOUT output from the OP-AMP is applied to the bias-voltage generator (
15
of
FIG. 3
) as a contrast controlling voltage. The rated contrast controlling voltage VOUT applied to the bias-voltage generator
15
ranges from −8 to −11 V, for example.
According to the aforementioned contrast control circuit for the conventional display apparatus, since an OP-AMP which is commercially available at high cost is necessarily used, the manufacturing cost increases. Also, circuit stability cannot be improved and power consumption increases.
SUMMARY OF THE INVENTION
To solve the above problems, it is an object of the present invention to provide a contrast control circuit for a display apparatus, which can reduce the manufacturing cost and power consumption and can improve circuit stability.
Accordingly, to achieve the above object, there is provided a contrast control circuit for a display apparatus, which receives a first voltage of positive polarity adjusted by a user, and outputs a second voltage of negative polarity for controlling the contrast of a display panel. The contrast control circuit includes third and fourth resistors, a first transistor, a second transistor, serially connected resistors and a Darlington circuit. The third and fourth resistors each have one end connected to the terminals of a positive supply voltage. The first transistor has its emitter connected to the other end of the third resistor and its base to which the first voltage is input. The second transistor has its emitter connected to the other end of the third resistor, its base connected to the other end of the fourth resistor, and its collector connected to a terminal of a negative supply voltage. The serially connected resistors have one end connected to the base of the second transistor and the other end connected to the output terminal of the second volt age. The Darlington circuit, constituents of which are connected between each of the output terminal of the second voltage, the collector of the first transistor and the terminal of the negative supply voltage, operates such that the absolute value of the second voltage is inversely proportional to the absolute value of the first voltage.
In the contrast control circuit for a display apparatus according to the present invention, the voltage of the base terminal of the second transistor is equal to the first voltage and the absolute value of the second voltage is inversely proportional to the absolute value of the first voltage. Accordingly, the second voltage can be generated by the first voltage, without using an operation amplifier which is commercially available at high cost. Thus, the manufacturing cost and power consumption can be reduced, and the circuit stability can be improved.
Preferably, at least one of the fourth resistor and the serially connected resistors is a thermistor. Accordingly, the circuit stability depending on temperature can be more effectively improved.
REFERENCES:
patent: 3649847 (1972-03-01), Limberg
patent: 3851094 (1974-11-01), Sasabe et al.
patent: 3914545 (1975-10-01), Engel
patent: 5049834 (1991-09-01), Kasai
patent: 5250937 (1993-10-01), Kikuo et al.
patent: 5724519 (1998-03-01), Kato et al.
patent: 5966110 (1999-10-01), Van Zalinge
Dharia Prabodh M.
Leydig , Voit & Mayer, Ltd.
Mengistu Amare
Samsung SDI & Co., Ltd.
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